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BACKGROUND The effect of a diet containing 15% grape pomace ( GP) on the general health status and milk quality of dairy cows was assessed by plasma biochemistry and total polyphenol ( TP) content, milk polyphenols, milk composition and milk protein fractions. RESULTS Among the polyphenols measured by liquid chromatography-mass spectroscopy in GP, in feed containing GP ( GP+) or not containing GP ( GP-), gallic acid and epicatechin were present in the highest concentrations (67.58 and 19.23 µg mL−1, respectively). Higher amounts of TP were also detected in the blood plasma of GP+ cows (114.06 and 83.93 mg GAE L−1, respectively) but not in their milk (233.17 and 245.75 mg GAE L−1, respectively). Also a significant increase was found for lactose and β-lactoglobulin, although there was no effect on α-lactalbumin, albumin, secretory components and caseins. CONCLUSION Inclusion of 15% GP in the diets of dairy cows is beneficial for overall normal blood constituent metabolism and helps to maintain cow health. The milk of cows fed with a GP diet preserves the normal levels of fat, protein and caseins, and has increased levels of components that make this milk a versatile ingredient material for the food industry (e.g. model whey powders, stability of lactose-rich powders). © 2016 Society of Chemical Industry [ABSTRACT FROM AUTHOR]